Endothelin (ET) is a 21 amino acid peptide that is produced by endothelial cells. ET is produced by enzymatic cleavage of a Trp-Val bond in the precursor peptide big endothelin (Big ET). This cleavage is caused by an endothelin converting enzyme (ECE). Endothelin has been shown to constrict arteries and veins, increase mean arterial blood pressure, decrease cardiac output, increase cardiac contractility in vitro, stimulate mitogenesis in vascular smooth muscle cells in vitro, contract non-vascular smooth muscle including guinea pig trachea, human urinary bladder strips and rat uterus in vitro, increase airway resistance in vivo, induce formation of gastric ulcers, stimulate release of atrial natriuretic factor in vitro and in vivo, increase plasma levels of vasopressin, aldosterone and catecholamines, inhibit release of renin in vitro and stimulate release of gonadotropins in vitro.
It has been shown that vasoconstriction is caused by binding of endothelin to its receptors on vascular smooth muscle (Nature 332 411 (1988), FEBS Letters 231 440 (1988) and Biochem. Biophys. Res. Commun. 154 868 (1988)). An agent which suppresses endothelin production or an agent which binds to endothelin or which inhibits the binding of endothelin to an endothelin receptor will produce beneficial effects in a variety of therapeutic areas. In fact, an anti-endothelin antibody has been shown, upon intrarenal infusion, to ameliorate the adverse effects of renal ischemia on renal vascular resistance and glomerular filtration rate (Kon, et al., J. Clin. Invest. 83 1762 (1989)). In addition, an anti-endothelin antibody attenuated the nephrotoxic effects of intravenously administered cyclosporin (Kon, et al., Kidney Int. 37 1487 (1990)) and attenuated infarct size in a coronary artery ligation-induced myocardial infarction model (Watanabe, et al., Nature 344 114 (1990)).
Clozel et al. (Nature 365: 759-761 (1993)) report that Ro 46-2005, a nonpeptide ET-A/B antagonist, prevents post-ischaemic renal vasoconstriction in rats, prevents the decrease in cerebral blood flow due to subarachnoid hemorrhage (SAH) in rats, and decreases MAP in sodium-depleted squirrel monkeys when dosed orally. A similar effect of a linear tripeptide-like ET-A antagonist, BQ485, on arterial caliber after SAH has also been recently reported (S. Itoh, T. Sasaki, K. Ide, K. Ishikawa, M. Nishikibe, and M. Yano, Biochem. Biophys. Res. Comm., 195: 969-75 (1993). These results indicate that agents which antagonize ET/ET receptor binding will provide therapeutic benefit in the indicated disease states.
Agents with the ability to antagonize ET/ET receptor binding have been shown to be active in a number of animal models of human disease. For example, Hogaboam et al (EUR. J. Pharmacol. 1996, 309, 261-269), have shown that an endothelin receptor antagonist reduced injury in a rat model of colitis. Aktan et al (Transplant Int 1996, 9, 201-207) have demonstrated that a similar agent prevents ischemia-reperfusion injury in kidney transplantation. Similar studies have suggested the use of endothelin antagonists in the treatment of angina, pulmonary hypertension, Raynaud's disease, and migraine. (Ferro and Webb, Drugs 1996, 51, 12-27).
Abnormal levels of endothelin or endothelin receptors have also been associated with a number of disease states, including prostate cancer (Nelson et al, Nature Medicine 1995, 1, 944-949), suggesting a role of endothelin in the pathophysiology of these diseases.
Wu-Wong et al (Lfe Sciences 1996, 58, 1839-1847) have shown that both endothelin and endothelin antagonists bind tightly to plasma proteins, e.g., serum albumin. This plasma protein binding can decrease the effectiveness with which the antagonists inhibit endothelin's action. Thus, endothelin antagonists with reduced plasma protein binding may be more effective than highly bound congeners.